1. Field of the Invention
The present invention generally relates to methods for driving light deflectors whereby the direction of outgoing light against incident light is changed. For example, the present invention can be used for image forming devices such as electrophotographic type printers or copiers, or projection type image display apparatuses such as projectors or digital theater systems.
2. Description of the Related Art
As an optical switch device using an electrostatic force, a device wherein a cantilever is deformed by an electrostatic force so that a reflection direction of light is changed (switched) and a light deflection system using the device were published by K. E. Petersen in 1977. See “Applied Physic Letters, Vol. 31, No. 8, page 521 to page 523”, Japanese Patent No. 2941952, and Japanese Patent No. 3016871. In addition, D. M. Bloom et al. published an element wherein a diffraction grating is driven by an electrostatic force so that optical switching is performed. See “Optical Letters, Vol. 7, No. 9, page 688 to page 690”.
Furthermore, Tibor et al. suggested a device wherein a digital micro mirror device is provided one-dimensionally or two-dimensionally, as an image device using a light deflection system. See Japanese Laid Open Patent Application No. 6-138403.
In addition, L. J. Hornbeck published a digital micro mirror device of a torsion beam type or a cantilever beam type as an element structure of the digital micro mirror device. See “Proc. SPIE Vol. 1150, page 86 to page 102, 1989”. In the digital micro mirror device of the torsion beam type or the cantilever beam type which was published by L. J. Hornbeck, as well as the present invention, a mirror part is inclined. However, the digital micro mirror device of the torsion beam type or the cantilever beam type which was published by L. J. Hornbeck is different from the present invention in that the digital micro mirror device of the torsion beam type or the cantilever beam type which was published by L. J. Hornbeck has a structure wherein the mirror part has at least one or more fixed ends.
Furthermore, Gelbart Daniel suggested an element wherein both ends fixed type beam is deformed so as to have a cylindrical shape configuration and light deflection is performed at high speed. See Japanese Laid Open Patent Application No. 2000-2842.
L. J. Hornbeck et al. published a projection type image display apparatus, wherein a plurality of torsion beam type optical switch devices are provided two-dimensionally and optical signals corresponding to image information of respective pixels are led to a projection lens if necessary, so that the image is displayed, that is conventional art which is used for an actual product as an application of the optical switch device at present. See “A MEMS-Based Projection Display Proceedings Of The IEEE. Vol. 86, No. 8, August 1998, page 1687 to page 1704”. According to “A MEMS-Based Projection Display Proceedings Of The IEEE. Vol. 86, No. 8, August 1998, page 1687 to page 1704”, one light source is used and light outgoing from the light source passes a color wheel which rotates so as to convert in turn to lights of three color, red, green, and blue. After that, the light goes into one chip which is an arrayed optical switch device and is reflected if necessary. As a result of this, the optical signals corresponding to image information of respective colors, red, green, and blue, are led in order so that the image is displayed. By using the above-mentioned system, it is possible to display the image by only one light source and one chip. Hence, it is possible to manufacture the projection type image display apparatus at relatively low cost.
Furthermore, another system with regard to projection by the projection type image display apparatus using the above-mentioned optical switch device was introduced. See “Using ZEMAX Image Analysis and user-defined surfaces for projection lens design and evaluation for Digital Light Processing™ projection systems, Optical Engineering, Vol. 39 No. 7, July 2000, page 1802 to page 1807”. More specifically, “Using ZEMAX Image Analysis and user-defined surfaces for projection lens design and evaluation for Digital Light Processing™ projection systems, Optical Engineering, Vol. 39 No. 7, July 2000, page 1802 to page 1807” introduced a technology whereby one light source is used and light outgoing from the light source passes through a TIR (Total Internal Reflection) PRISM. After that, the light passes through a COLOR PRISM which works for color separation and color composition so that color separation is performed and respective color lights enter into three chips. The lights are reflected in directions of objects if necessary. The lights pass through the COLOR PRISM again so as to be color-composed. The color composed light is led to the projection lens and thereby the image is displayed. By using the above-mentioned system, it is possible to display respective colors, red, green, and blue, simultaneously but not at low cost. Hence, it is possible to maximize the time for displaying respective colors at one frame and therefore it is possible to provide a projection type image display apparatus having high brightness.
The Japanese publication “Digital Micro Mirror Device Applied Physics Vol. 68, No. 3, 1999, page 285 to page 289” introduces a system of the projection type image display apparatus suggested by L. J. Hornbeck et al.
Japanese Laid Open Patent Application No. 2002-131838 introduces a projection apparatus as conventional art of the projection type image display apparatus using the optical switch wherein a diffraction grating is driven by an electrostatic force, published by D. M. Blooms. The projecting apparatus projects a light flux as an image on a screen by scanning by a scanning mirror. The light flux includes a vertical or horizontal line of image components formed by a color combining mechanism having a laser light source and a spatial modulator comprising a diffraction grating element, which modulates the phase, arranged in a one-dimensional line. However, since the laser light source is required from the perspective of the optical switch, the cost for manufacturing the projection type image forming device is made high.
The above-discussed optical switch using the cantilever beam or the cantilever type digital micro mirror device has weak points, that is, difficulty in ensuring stability of the beam and difficulty in making the response speed high. Furthermore, the torsion beam type digital micro mirror device also has a weak point in that mechanical strength of a hinge part (torsion beam) is degraded when used for a long period of time. In addition, the optical switch discussed in Japanese Patent 2941952 and Japanese Patent 3016871 has a weak point in that the wavelength of the incident light is limited. The element discussed in Japanese Laid Open Patent Application No. 2000-2842, namely an element whereby both ends fix beam is deformed in a cylindrical shape configuration by an electrostatic force at a space between electrodes, has an advantage in that it is possible to deform at high speed so that the response speed can be made high. However, in this element, since both ends are fixed, it is not possible to make the driving voltage as low as the device of the cantilever beam type or torsion beam type.
The above-discussed conventional art optical switches are shown in FIG. 1. More specifically, a torsion type is shown in FIG. 1-(a), a cantilever type is shown in FIG. 1-(b), and a both ends fixed beam type is shown in FIG. 1-(c). The above-mentioned optical switches include two or three electrodes. In the optical switches, different electric potentials are applied between electrodes facing each other at a plane surface so that an electrostatic force is applied. As a result of this, the mirror surface is changed and therefore light deflection is performed. On the other hand, in the light deflection device of the present invention, a plurality of electrodes such as four electrodes, are formed at a same plane surface and a plate member having a conductive layer which electrically floats is formed so as to face the electrodes. The plate member which electrically floats has optional electric potential by the application of different electric potentials between neighboring electrodes, so that an electrostatic force acts on the plate member. The plate member having a mirror surface is inclined and deformed against a fulcrum member as a center so that the light deflection operation is implemented.
There is a problem regarding an electrical charge stored at an insulating film between electrodes due to high driving voltage, which is a problem common to devices (hereinafter, “electrostatic device”) driven by an electrostatic force including the light deflection device of the present invention. Due to this electrical charge storage problem, the insulating film, formed so as to prevent the electrodes facing each other from shorting, stores electrical charges at a localized level in the vicinity of electrode interfaces in an electrical field of several mV/cm, and therefore it appears that the insulating film has electric potential. As a result of this, electrostatic attraction between the electrodes is reduced, the deformation is reduced, and stored electrical charge stays behind so that the deformation is generated even though the electric potential is provided between the electrodes. This causes an incorrect action of the light deflection device, for example.
For the above-discussed problems, there is a method to improve the electrical charge storage at the electrostatic device formed by two electrodes facing each other at the plane surface. See Japanese Laid Open Patent Application No. 7-214775, Japanese Laid Open Patent Application No. 7-214776, Japanese Laid Open Patent Application No. 7-214780, and Japanese Laid Open Patent Application No. 9-136413. In electrostatic actuating type printing apparatuses described in the Japanese Laid Open Patent Application No. 7-214775, Japanese Laid Open Patent Application No. 7-214776, Japanese Laid Open Patent Application No. 7-214780, and Japanese Laid Open Patent Application No. 9-136413, a vibration plate which is a common electrode is grounded. Also, an electric potential is applied to individual electrodes by mutually changing polarity in timing with ink discharge. In addition, a reversed polarity electric potential is applied to the individual electrodes at the time when the printing apparatus starts working so as to remove electrical charge stored at the insulating film.
However, the above-discussed driving method for controlling the electrical charge storage cannot be applied to a structure where the plate member which electrically floats is deformed due to an electrostatic attraction generated from an electrical field based on a difference of electric potentials applied to the plurality of electrodes formed at the same plane surface.